Field of the Invention
The present invention relates to a manufacturing method for a multi-bit memory cell with self-adjusting ONO regions.
In U.S. Pat. No. 5,768,192, a non-volatile memory is described in which electrons are trapped at a source region or a drain region respectively in a memory layer. The trapped electrons determine a threshold voltage of the transistor, which is configured as a semiconductor oxide nitride oxide semiconductor (SONOS) transistor. The presence of a charge at the source or drain respectively can be interpreted as a stored bit so that two bits can be stored in a cell of this kind. For programming, hot charge carriers are produced in the channel; the electrons are injected near to the drain region from the semiconductor material into the memory layer. In addition, a potential difference of typically 5 V is applied to a word line running via the gate in the direction from the source to the drain. The source region itself is connected to 0 V and the drain region, as a bit line, to 5 V. By reversing the applied voltage, charges can also be trapped in the source region. A potential difference of typically 1.2 V between the source and drain and a gate voltage between the threshold voltage in the unprogrammed state and the threshold voltage in the programmed state enable the bit stored on the source side to be read. A clearly negative potential on the gate and e.g. 5 V on the drain (word line almost current-less) allow erasure by forcing the trapped charge carriers back into the source or drain region, which is positive with respect to ground (GIDL, gate-induced drain leakage).
These days, in a highly integrated memory, the distance from the source to the drain is only 150 nm. If the memory chips are stored in unfavorable conditions (particularly at high temperatures, typically 85xc2x0 C., which can be reached in an automobile for example), the number of charge/discharge cycles that can be carried out without any problem (endurance) can be reduced if the trapped charge carriers no longer remain sufficiently localized in the dielectric nitride. It is then more difficult to read out separately the stored bits at source and drain.
In U.S. Pat. No. 5,877,523, a multilevel split gate flash memory cell is described in which an oxide layer and a polysilicon layer intended as a memory layer are applied and structured to form floating gate electrodes in two parts. The remaining parts are coated with a dielectric layer. Onto this is applied a conducting layer, which is structured to form a gate electrode. This is supplemented by implantations of a doping material to form the source and the drain.
In U.S. Pat. No. 5,969,383, a split gate memory device is described in which a series of layers of silicon dioxide, silicon nitride and silicon dioxide is provided over part of the channel region and over part of the drain region in each case, of which the series of layers over the channel is provided with a control gate electrode. The programming of the memory cell takes place at this point by trapping charge carriers in the silicon nitride layer. A selection gate electrode is disposed over a remaining part of the channel region.
In U.S. Pat. No. 5,796,140, a method for manufacturing a memory cell is described in which the source and the drain are formed as doped regions, separated from one another by a channel region. A memory layer intended for storing charge carriers is provided over these regions between the boundary layers and embedded therein, a gate electrode is applied, separated from the semiconductor material by a dielectric layer, whereby the memory layer, with the exception of regions which are located at the boundaries between the channel region and the source or drain region respectively, is removed.
In published, Japanese Patent Application JP 2000-58680, a semiconductor memory device is specified in which an oxide-nitride-oxide layer is also applied to the edges of the gate electrode.
It is accordingly an object of the invention to provide a method for manufacturing a multi-bit memory cell that overcomes the above-mentioned disadvantages of the prior art methods of this general type, which guarantees a larger number of charge and discharge cycles even under unfavorable conditions.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for manufacturing a memory cell circuit. The method includes providing a body being either a semiconductor body or a layer of semiconductor material, and providing a series of layers on the body. The series of layers include a first oxide layer, a memory layer disposed above the first oxide layer and intended for storing charge carriers, and a second oxide layer disposed above the memory layer. The memory layer outside a region intended for a memory cell is removed. An implantation of a doping material in the body is carried out for forming a source region and a drain region separated by a channel region and disposed below the series of layers. An auxiliary layer is provided outside the region occupied by the memory layer. The auxiliary layer has a recess in the region of the memory layer with sufficiently steep edges. Spacers are formed in the recess at the edges of the auxiliary layer. A part of the memory layer disposed between the spacers is removed. A dielectric layer is formed between the spacers. A gate electrode is formed on the dielectric layer and the dielectric layer and the gate electrode are structured. A conductor track is applied, the conductor track is electrically conductively connected to the gate electrode.
In the multi-bit memory cell manufactured according to the invention, the memory layer intended for trapping charge carriers at the source and the drain is limited to the edge region of the source region or drain region bordering the channel region. The memory layer is disposed between the boundary layers and embedded in a material with a higher energy band gap so that the charge carriers, which are trapped in the memory layer over the source region and over the drain region respectively, remain localized there.
A nitride is preferably considered for the material for the memory layer; an oxide is primarily suitable for the surrounding material. In the case of a memory cell in the silicon material system, the memory cell in this example is silicon nitride with an energy band gap of about 5 eV, the surrounding boundary layers are silicon oxide with an energy band gap of about 9 eV. The memory layer can be a different material with a smaller energy band gap than that of the boundary layers, whereby, for a good electrical confinement of the charge carriers, the difference in the energy band gaps should be as large as possible. Therefore, tantalum oxide, hafnium silicate or intrinsically conducting (undoped) silicon, for example, can be used in conjunction with silicon oxide as the material for the memory layer. Silicon nitride has a relative dielectric constant of about 7.9. The use of an alternative material with a higher dielectric constant (e.g.=15 . . . 18) permits a reduction in the overall thickness of the stack of layers provided for storage and is thus an advantage.
In the method, the memory layer provided for trapping the charge carriers is completely removed outside a region over that boundary of the source region and the drain region that, in each case, faces the channel region. After this, the gate oxide and the gate electrode or a conducting track intended as a word line are manufactured and structured and edges of the memory layer that are still free are embedded in the material of the boundary layers, preferably an oxide. By removing the memory layer over the channel region, the SONOS transistor cell manufactured in this way has memory regions over source and drain that are separated from one another.
In a particularly preferred exemplary embodiment, the gate oxide is not only manufactured on the semiconductor material of the channel region in the substrate but also in the vertical direction on the edges of the gate electrode so that, by this process, an additional electrical insulation of the adjacent memory layers is effected. The manufacture of a vertical oxide layer on the edges of the gate electrode also changes the distribution of the electrical field so that the hot electrons are accelerated more strongly in the direction of the memory layer and are trapped there. The operational characteristics (performance) of the memory cell, in particular when programming, are significantly improved by this process.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for manufacturing a multi-bit memory cell, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.